Resistor films of transition metal nitrides and method of forming



Nov. 3, 1970 J. R. RAIRDEN m 3,537,891

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United States Patent 01 fice 3,537,891 Patented Nov. 3, 1970 3,537,891RESISTOR FILMS OF TRANSITION METAL NITRIDES AND METHOD OF FORMING JohnR. Rairden HI, Niskayuna, N.Y., assignor to General Electric Company, acorporation of New York Filed Sept. 25, 1967, Ser. No. 670,091 Int. Cl.C23c 11/08, 11/14 US. Cl. 117-215 10 Claims ABSTRACT OF THE DISCLOSUREResistor thin films of nitrides ofthe groups IV and V transition metalsare formed by the reactive evaporation of the chosen transition metal ina nitrogen atmosphere between 5 10 to torr. The substrate upon which theresistor film is deposited is preheated to a temperature of about 420 C.and the transition metal is deposited at a rate of approximately 100 A.per minute atop the substrate. Upon completion of the deposition of thenitride resistor film to the desired thickness, evaporation of the metalis discontinued and the resistor film is heat treated at a temperatureof approximately 400 C. for 8 minutes in a nitrogen environment greaterthan 0.5 torr to produce a resistor film having a zero (p.p.m./ C.)temperature coefiicient of resistance between C. and 125 C. Tantalum andniobium nitride resistor films formed by reactive evaporation have beenfound to be very abrasion resistant exhibiting no change in resistanceafter traversal of 10,000 revolutions with a carbonaceous spring biasedcontact while substantial variations in resistance were noted for bothnickel-chrome and zirconium nitride films traversed by the identicalcontact.

This invention relates to thin film resistors and in particular to thinfilm transition metal nitride resistors formed by reactive evaporationin a low pressure nitrogen atmosphere.

Among the desirable characteristics for a resistor thin film fabricatedinto printed circuitry are high resistivity, low coefficient ofresistance and stability and thin films of tantalum nitride formed bythe reactive sputtering of tantalum in a partial nitrogen pressure ofapproximately 10- torr heretofore have been known to possess theseresistor characteristics. For example, tantalum nitride films producedby reactive sputtering have been known to exhibit a specific resistivityin the order of 200 to 250 micro ohm centimeters, a temperaturecoefiicient of resistance of approxmiately zero (p.p.m./ C.) between 25C. and 125 C. and excellent stability upon heat treatment of theresistor film in the presence of air at temperatures in the range of 250to 400 C.

I have discovered however that resistor thin films formed by thereactive evaporation of a transition metal in 'a low pressure nitrogenatmosphere, e.g. between 5 X10 to 10* torr, exhibit a specificresistivity substantially higher (approximately 67% for comparabletantalum nitride resistor films) than the specific resistivityobtainable by reactive sputtering. The exact cause of this phenomena isunknown. However one possible cause of the variation in resistivitybetween reactively sputtered re sistor films and reactively evaporatedresistor films may be postulated by considering the specific resistivityof a resistor film to be a composite of the resistivity produced bylattice vibration, the resistivity due to surface function and theresistivity due to defect or interruptions in the repetitive geometry ofthe crystalline structure. Because the surface function resistivity isprimarily a function of the thickness of the film and because theresistivity due to lattice vibration is primarily a function of thecomposition of the material (both of which film characteristics areassumed to be identical for comparable resistor films formed by reactivesputtering and reactive evaporation), the specific resistivityvariations between comparable resistor film-s formed by the two methodswould appear to result from a divergence in the repetitive crystallinestructure of the two films. Thus the crystalline structure of resistorfilms may vary dependent upon the process employed in the reactiveformation of the resistor thin films.

The resistor elements formed by the reactive evapora tion process ofthis invention also has been found to exhibit a fine grain size,excellent wear resistance, relative immunity to corrosion, temperaturestability and high resistance.

It is therefore an object of this invention to provide a novel method ofproducing superior resistor films of transition metals.

It is also an object of this invention to provide resistor films oftransition metal nitrides having high resistivity, fine grain size,temperature stability and superior wear resistance.

It is a further object of this invention to provide a potentiometerhaving a thin film resistor element exhibiting superior wear resistance.

These and other objects of this invention generally are achieved by thinfilm resistors having a nitride of a transition metal chosen from thegroup consisting of niobium, tantalum, vanadium, titanium, zirconium andhafnium positioned atop a non-conductive substrate where the transitionmetal nitride is formed by the reactive evaporation of the chosentransition metal in a low pressure nitrogen atmosphere betweenapproximately 5 10- to 10 torr. Preferably these thin film resistors areformed by positioning a substrate and a transition metal selected fromthe group consisting of niobium, tantalum, vanadium, titanium, zirconiumand hafnium within an enclosed chamber and introducing a quantity ofnitrogen bearing gas into the chamber to produce a low pressure nitrogenatmosphere in a range between approximately 5X10" to 10- torr. After thesubstrate is heated to a temperature in excess of 30 C., the transitionmetal is evaporated in the nitrogen atmosphere and a thin film nitrideresistor is condensed atop the substrate. To increase the resistivityand to decrease the temperature coefficient of resistance of theresistor film, the film is baked in a nitrogen atomsphere above 0.5 torrat elevated temperatures above 30 C. prior to removal of the resistorfilm from the nitrogen atomsphere of the enclosed chamber. By admittingair into the chamber after the post-deposition baking of the film whilethe resistor film is still warm, e.g. at approximately 245 C., a thinoxide stabilizing layer is formed atop the resistor film.

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself, however,both as to organization and method of operation, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description taken in conjunction with the accompanyingdrawings in which:

FIG. 1 is an isometric section of an apparatus suitable for formingresistor thin films in accordance .with this invention,

FIG. 2 is a graph depicting the variation of resistance with nitrogenpressure for niobium nitride resistor films formed by reactiveevaporation in a low pressure nitrogen atmosphere,

FIG. 3 is a graph depicting the variation of resistance with temperaturecoeflicient of resistance for niobium nitride films formed by reactiveevaporation,

FIG. 4 is a graph depicting the variation of nitrogen pressure withtemperature coefficient of resistance for tantalum nitride resistorfilms formed by the method of this invention,

FIG. 5 is a graph depicting the variation of temperature coeflicient ofresistance with resistance for tantalum nitride resistor films depositedutilizing the apparatus of FIG. 1,

FIG. 6 is a graph depicting the variation of resistance with temperaturecoefficient of resistance for zirconium nitride resistor films depositedby reactive evaporation in a low pressure nitrogen atmosphere,

FIG. 7 is a graph depicting the variation of resistance with temperaturecoefiicient of resistance for hafnium nitride resistor films depositedin accordance with this invention, and

FIG. 8 is a graph depicting the variation of resistance with temperaturecoefficient of resistance for both titanium nitride and vanadium nitrideresistor films deposited by reactive evaporation in a low pressurenitrogen atmosphere.

An apparatus suitable for the deposition of transition metal nitridethin films in accordance with this invention is depicted in FIG. 1 andgenerally includes an evaporation chamber 10 having an electron beamsource 12 for the evaporation of a transition metal source 14. Suitablemeans, such as conduit 16, are provided for the controlled admission ofa nitrogen bearing gas 18, e.g. nitrogen or ammonia, into the enclosedchamber and the electron beam evaporated transition metal reacts withthe nitrogen within the enclosed chamber to deposit a transition metalnitride resistor film upon substrate 20.

Evaporation chamber 10 generally includes a water cooled stainless steelenvelope 22 seated upon a circular base 24 and a gasket is providedintermediate the vertical sidewalls of the chamber and the base to sealthe interior of the chamber. Approximately centrally positioned withinbase 24 is a circular aperture 32 which communicates the interior of thechamber with vacuum pump 34 through lines 36 and 38 for the evacuationof the sealed chamber while an enclosed liquid nitrogen trap 40 ispositioned between evacuation lines 36 and 38 to effectively seal thechamber from contamination during operation.

A second aperture 42 within the base 24 communicates the interior ofevaporation chamber 10 with a nitrogen bearing gas source 18 throughconduit 16 and an automatic pressure control valve 44 is inserted withinconduit 16 intermediate nitrogen bearing gas source 18 and aperture 42to control the flow of low pressure nitrogen into the chamber prior toand during reactive evaporation. An ionization gauge 46 mounted withinevaporation chamber 10 and communicated to automatic pressure controlvalve 44 by electrical lead 48 passing through an aperture in the baseof the chamber serves to automatically govern the operation of thecontrol valve thereby regulating the pressure of nitrogen bearing gaswithin the chamber to produce a nitrogen pressure between 5X10 to 10-torr during evaporation of source 14.

A water cooled crucible 50 is approximately centrally positioned uponbase 24 and is supported in a slightly elevated position above the baseby machine screws 52 extending through suitable apertures in a pair ofvertically extending, slightly convergent magnetic pole pieces 54.Crucible 50 is interiorly bored (not shown) to allow a flowing coolantto function as a heat transfer agent for the crucible. The transitionmetal source 14, from which the resistor film is fabricated, ispositioned within the cup 56 of the water cooled crucible at a locationalong the arcuate path of the electron beam produced by electron beamsource 12. Transition metal source 14 can be any metal selected from thegroup consisting of niobium, tantalum, vanadium, titanium, zirconium orhafnium and may have the physical configuration of an ingot.

The heat source used in the preferred method of this invention toevaporate a portion of transition metal source 14 is a beam generated byelectron beam source 12 which source is depicted as a transverseelectron gun fixedly secured to the sidewall of crucible 50. Theelectron gun generally includes a cathode 60 suitably energized, e.g.with a DC potential 58 of approximately 10 kv. by electrical leads 62,for the generation of electrons and a centrally apertured anode 64 atground potential for acceleration of the generated electrons in agenerally vertical stream within evaporation chamber 10. When theelectron stream generated by the electron gun passes outside thegenerally enclosed gun into the magnetic field produced by magnetic polepieces 54, the stream is deflected in an arcuate direction to impingethe electrons upon transition metal source 14 within cup 56'. For theevaporation of the transition metal source, a 3.2 kw. deposition powerwas found to provide suflicient bombardment of the transition metalsource to deposit the metal nitride at an acceptable rate ofapproximately 100 A. per minute upon substrate 20 situated 14 inchesfrom transition metal source 14.

The distance from source 14 to substrate 20 preferably is fixed relativeto the nitrogen gas pressure in the chamher, as controlled by pressurecontrol valve 44, to produce one or more collisions between theevaporated transition metal in the gas phase and a nitrogen moleculebefore deposition of the metal upon the substrate. To effectuate thisresult, a nitrogen pressure of 1 10- torr generally requires a minimumsource to substrate distance of 5 cm. with a tenfold decrease innitrogen pressure, e.g. to 1 10- torr, requiring approximately a tenfoldincrease in source tosubstrate distance, e.g. to 50 cm.

The substrate 20 upon which the transition metal nitride resistor filmwill be deposited lies within a ledge of rectangular frame 68 whichframe is supported in an elevated position by an angularly shaped brace70. Substrate 20 may be any non-conductive heat resistant material, e.g.soda lime glass, quartz, mica or magnesium oxide, and the lower face ofthe substrate is situated in a generally confronting attitude withtransition metal source 14. A tungsten wire heater 72 and a heatreflector 74 are positioned slightly above the substrate face remotefrom the transition metal source and a platinum/platinum-rhodiumthermocouple 76 is seated along the edge of the upper face of thesubstrate to produce a visual indication of the substrate temperatureupon temperature gauge 88. Energization of heater 72 is provided by a40-volt, 5 ampere source of alternating current 80 connected to theheater through a switch 82 and electrical leads 78.

A mask 84 apertured to the dimension desired for the thin film resistor,e.g. 1 mm. x 10 mm, is positioned upon the upper outer extension ofpivotal rod 86 and is rotatable by suitable external means (not shown)to a position in an underlying relationship with substrate 20 to controlthe geometry of the transition metal resistor thin film deposited uponthe substrate.

In the operation of this invention, substrate 20 is cleaned in asuitable manner, for example, soda lime glass microscopic substratespreferably are cleaned by boiling the substrate in water containing adetergent, successively rinsing the substrate in cold water, hotdeionized water, and isopropyl alcohol and then drying the substrate inhot vapors of isopropyl alcohol. After cleaning and drying, thesubstrate is positioned within frame 68 which is situated at a suitabledistance, e.g. 14 inches, above a highly pure transition metal source 14seated in cup 56 of crucible 50. Thermocouple 76 then is positionedalong the upper edge of the substrate remote from source 14 to measurethe substrate temperature and evaporation chamber 10 is evacuated to alow pressure of approximately 10 torr by pump 34. Upon evacuation of theevaporation chamber, the chamber is purged with nitrogen bearing gas 18at pressures preferably in a nitrogen range of 1X10 torr to 8X10 torrfor a period of approximately 5 minutes and mask 84 is positioned belowthe face of the substrate to permit the deposition of the resistor filmupon a selected area of the substrate. Electron beam source 12 then isenergized to produce an evaporation of transiton metal source 14 at asuflicient rate to deposit approximately 50 to 400 A. per minute of themetal nitride upon substrate 20 with nitrogen pressure in theevaporation chamber preferably being maintained in a range betweenapproximately 5x10 torr to torr during the evaporation by ionizationgauge 46 and pressure control valve 44. To assure superior resistivecharacteristics in the deposited film, the substrate preferably ispreheated to and maintained at a temperature in a range between 420 C.to 465 C. during the deposition of the transition metal. When substratesmore refractory than soda lime glass are employed, the substrate mayadvantageously be heated to temperatures in excess of 465 C. Uponcompletion of the deposition of the transition metal nitride resistorfilm to the desired thickness, electron beam source 12 is deenergizedand the deposited resistor film is baked at an elevated temperature,e.g., preferably above 370 C., for a period of approximately minutes ina nitrogen atmosphere greater than 0.5 torr. After the post-depositionbaking of the resistor film is completed, the resistor film is allowedto cool to about 245 C. whereupon air is admitted to chamber 10 and anoxide coating is formed upon the resistor film. The resistor film thenis temperature cycled between C. and 125 C. to produce a high specificresistivity low thermal coefficient of resistance transition metalnitride resistor film.

A more complete understanding ofthe reactive evaporation method of thisinvention and the unique properties of the transition metal nitrideresistor films formed thereby can be better exemplified by the followingspecific examples of the reactive evaporation of various transitionmetals.

EXAMPLE 1 After a cleaned soda lime glass slide substrate was positionedwithin rectangular frame 68 and a source of pure niobium seated within'cup 56 of water cooled crucible 50 at a distance 14 inches from thesubstrate, switch 82 was closed to energize heater 72 with electricpower from source 80 and the substrate temperature was raised toapproximately 420 C. The closed chamber then was evacuated toapproximately 5X10 torr by evacuation pump 34, whereupon pressurecontrol valve 44 was activated by ionization gauge 46 to admit andmaintain nitrogen at a pressure of8 10" torr within the chamber. Afterenergization of electron beam source 12 at a sufficient potential toevaporate the niobium source, niobium nitride was deposited through a 1x 10 mm. mask for a -minute period at a rate of approximately 100 A. perminute upon the soda lime glass substrate whereupon energization ofelectron beam source 12 was terminated. Upon completion of thedeposition of the niobium nitride resistor film, nitrogen was admittedto the chamber to raise the nitrogen pressure in the chamber greaterthan 0.5 torr. and the resistor film was baked at a temperature of 400C. in the relatively high pressure nitrogen atmos phere for a period of15 minutes. The baked niobium nitride film then was permitted to cool toapproximately 245 C. whereupon air was admitted to the chamber to forman oxide layer atop the warm niobium nitride resistor film.

Upon complete cooling of the resistor thin film, the slide was removedfrom frame 68 and indium solder dots were placed along the surface ofthe resistor film to permit four probe resistance measurements whichmeasurements disclosed the specific resistivity of the sample to be 580micro-ohm-centimeters. The resistor film stabilized upon the firstthermal cycle, e.g. from 25 to 125 C., and the temperature coefiicientof resistance of the deposited film measured zero (p.p.m./ C.) between25 C. and 125 C. subsequent to the initial thermal cycling.

When niobium nitride resistor films were deposited under conditionsidentical to those heretofore described in Example 1 except for thepre-heating of the substrate, the niobium nitride resistor filmsdeposited on the unheated substrate were found to have a high negativethermal coefiicient of resistance and a very high resistance whichincreased markedly upon thermal cycling. Thus, to effectively producethin niobium nitride resistor films suitable for thin film circuitryaccording to the method of this invention, the substrate should beheated to a temperature at least in excess of 30 C. prior to theinitiation of, and during, evaporation. When a blank shield is employedto protect the substrate from the initial depositions duringevaporation, substrate preheating by an external source may not benecessary and preheating of the substrate to a temperature in excess of30 C. may be accomplished by heat generated during the evaporationprocess.

The importance of the post-heat treatment of the deposited niobiumnitride resistor films in relatively high pressure nitrogen, e.g. apressure greater than 500 microns of mercury, immediately afterdeposition was shown by the fact that 5 niobium nitride resistor filmspost-heat treated in the relatively high pressure nitrogen atmosphere attemperatures in excess of 380 C. exhibited resistance values in execessof 265 ohms per square and Zero (p.p.m./ C.) temperature coefficients ofresistance between 25 C. and 125 C. Two films deposited under theidentical conditions but heat treated and subsequently cooled in highvacuum (approximately 2 10 torr) exhibited a low resistance and adecidedly positive temperature coefiicient of resistance.

The admission of air into evaporation chamber 10 while the niobiumnitride resistor film was cooling, e.g. at 245 C., after the post-heattreatment in the relatively high pressure nitrogen atmosphere was foundto produce stability in the films on the first thermal cycle between 25C. and 125 C. while resistor films completely cooled in a nitrogenatmosphere generally required a plurality of temperature cycles beforestability was obtained. The rapid stabilization of niobium nitrideresistor films partially cooled in air appears probably to be due to alimited oxidation on the resistor film surface and perhaps in the grainboundaries of the film.

An increase in the resistance of the deposited niobium nitride resistorfilms with increasing nitrogen pressure in the evaporation chambergenerally was observed and is depicted in FIG. 2. It will be noted fromthe kinetic theory of gases that for nitrogen pressure readings inexcess of approximately 1.5 l0 torr, the mean free path of the vaporizedniobium atoms is less than the source to substrate distance of 14inches. This fact in conjunction with the graph of FIG. 2 indicates thata gas phase reaction is required for the formation of high ohmic valueniobium nitride resistor films. Thus for optimum resistor films, thesource to substrate distance is fixed relative to the nitrogen gaspressure in the chamber to produce at least one collision betweenniobium atoms in the gas phase and a nitrogen molecule prior todeposition of the niobium nitride resistor film on the substrate.

The effect of deposition rate upon the resistance properties of niobiumnitride resistor films formed by reactive evaporation was examined byvarying the deposition rates from approximately A. per minute toapproximately 1200 A. per minute. The deposition time of each reactiveevaporation also was varied to produce resistor films of substantiallythe same thickness from each deposition. Subsequent resistancemeasurements of the various resistor films indicated all films to haverelatively identical resistances.

A graph, illustraetd in FIG. 3, of resistance against temperaturecoefficient of resistance values for reactively evaporated niobiumnitride resistor films indicates thick resistor films (e.g. lowresistance films) to have a positive temperaure coefficient ofresistance with thin resistor films (e.g. high resistance films) havinga negative temperature coeflicient of resistance. This thickness effectsuggests a layered gradiation of the electrical properties in the filmwith the portion of the film proximate the substrate having a negativetemperature coeificient of resistance and the upper layers of the filmhaving a positive temperature coefficient of resistance. This layeredeffect of the resistor films also was indicated by anodization of arelatively thick resistor film to gradually convert the outer layers ofthe film to non-conducting oxides. During the anodization process, thetemperature coefficient of resistance was observed to go from positiveto negative and a film with a near zero temperature coeificient ofresistance was obtained upon a balancing of the positive and thenegative temperature coefficient of resistance portions of the films.

X-ray diffraction of the several niobium nitride resistor filmsindicated that hexagonal films of Nb N having a small crystallite size,e.g. less than 1000 A., are deposited during the reactive evaporationprocess. A preferred orientation of (1120) planes parallel to thesubstrate was observed for resistor films deposited at approximately 100A. per minute while films deposited at a faster deposition rateexhibited little or no orientation.

EXAMPLE 2 After a clean soda lime glass slide substrate was placedwithin frame 68 and a tantalum source 14 positioned within cup 56 ofwater cooled crucible 50 approximately 14 inches from the source,envelope 22 was seated upon base 24 and heater 72 was energized throughswitch 82 to raise the temperature of the substrate to 465 C. Thepressure of the evaporation chamber then was reduced by evacuation pump34 to approximately 5 10 torr whereupon nitrogen from source 18 wasadmitted to the chamber to raise the nitrogen pressure in the chamber toapproximately 6X10- torr and electron beam source 12 was energized toevaporate the tantalum source at a rate sufficient to depositapproximately 250 A. per minute of tantalum nitride upon the glasssubstrate. Evaporation of the tantalum source was continued until atantalum nitride resistor film 3300 A. thick was deposited atop thesubstrate. After completion of the resistor film deposition, nitrogenpressure in the chamber was raised above 0.5 torr and heating of theresistor film was continued at 465 C. for a period of 15 minutes beforethe resistor was allowed to cool to room temperature and removed fromthe evaporation chamber. Four indium solder contacts then were made tothe tantalum nitride resistor film to permit resistance measurements bythe four probe technique and the resistor film was temperature cycledbetween C. and 125 C. for approximately four cycles. Subsequentresistance measurements of the film indicated a zero (p.p.m./ C.)temperature coefficient of resistance be tween 25 C. and 125 C. and aspecific resistivity of approximately 420 micro ohm centimeters, e.g.approximately 67% higher than the specific resistivity of tantalumnitride resistor films formed by reactive sputtering. Tantalum nitrideresistor films which were not post-heat treated in the nitrogenatmosphere of the chamber were found to exhibit a positive temperaturecoefficient of resistance and a smaller specific resistivity.

The effect of nitrogen gas pressure upon film electrical properties wasfound to be slightly different than that observed for niobium films. Ina pressure range between 1X10 torr to 8 10 torr, the resistance valuesof the film for various pressures were found to be approximatelyidentical. However, the temperature coefficient of resistance values ofthe tantalum nitride films (shown in FIG. 4) systematically become morepositive with increasing nitrogen pressures. As will be observed fromFIG. 5, the trend of tantalum nitride resistor films deposited byreactive evaporation is for low resistance value films, e.g. relativelythick films, to have a positive temperature coefiiicent of resistancewhile high resistance films, e.g. thin films, have a negativetemperature coefficient of resistance.

X-ray diffraction analysis of a film 3300 A. thick formed by thereactive evaporation method heretofore described revealed only hexagonaltantalum nitride to be present within the resistor. This configurationdiffers from tantalum nitride resistor films formed by reactivesputtering under similar conditions wherein tantalum nitride of a cubicstructure is favored by sputtering in nitrogen pressures of 1 10- mm. ofmercury and higher. The approximate means particle size in the tantalumnitride resistor films formed by reactive evaporation was found to beapproximately 130 A. and there was some preferred orientation of (0001)planes parallel to the substrate surface upon which the resistor filmwas deposited. No load resistance life tests of the tantalum nitrideresistor films of this invention disclosed the lower resistance films,e.g. films having a resistance less than 250 ohms/ square, to be morethermally stable than the higher resistance, e.g. thinner films.Admission of air to the evaporation chamber during the cooling cyclewhen the tantalum nitride resistor reached approximately 2450 C.permitted the resistor to be stabilized in a single temperature cycle.

EXAMPLE 3 After a zirconium source and a cleaned soda lime substratewere placed in evaporation chamber 10, the chamber was evacuated toapproximately 5 10 torr by evacuation pump 34 whereupon nitrogen fromnitrogen bearing gas source 18 was admitted to the evaporation chamberto rise the nitrogen pressure of the chamber to approximately 8 x10torr. The substrate then was heated by alternating current source to atemperature of approximately 465 C. and evaporation of the zirconiumsource was initiated by energizing electron beam source 12. Theevaporation of the zirconium source was continued at a rate to produce adeposition of 200 to 300 A. per minute of zirconium nitride upon thesubstrate which substrate was positioned 14 inches from the source.After deposition of a film approximately 7000 A. thick, evaporation ofthe zirconium source was terminated and the deposited zirconium nitrideresistor film was post-heated at 465 C. for a period of 15 minutes in anitrogen atomsphere greater than 0.5 torr. Upon subsequent cooling ofthe resistor film, four indium solder dots were deposited atop thezirconinum nitride resistor film to permit four probe resistivemeasurements and the resistor film was thermal cycled between 25 C. andC. to stabilizer the film. Subsequent measurement of resistance valuesof the film disclosed a resistance of approximately 305 ohms per square.The temperature coefiicient of resistance of the film after themalcycling measured zero (p.p.m./ C.) between 25 C. and 125 C. X-raydiffraction analysis of an approximately 7000 A. thick resistor filmshowed only ZrN lines to be present with a preferred 0rientation of(111) planes parallel to the substate surface upon which the resistorfilm was deposited.

As can be observed from the graph of FIG. 6, a zero (p.p.m./ C.)temperature coefficient of resistance between 25 C. and 125 C. was foundin reactively evaporated zirconium nitride resistor films over a widerange (from 400 ohms/ sq. to 1400 ohms/ sq.) of resistance values.

9 EXAMPLE 4 After positioning a clean soda lime glass slide substratewithin frame 68, a hafnium source was positioned in the cup 56 ofcrucible 50 and envelope 22 of evaporation chamber 10 was positioned onbase 24. The substrate then was heated to a temperature of approximately465 C. by heater 72 and the evaporation chamber was evacuated to apressure of approximately 5X10 torr by pump 34. Upon exhausting of theair from the evaporation chamber, nitrogen from nitrogen bearing gassource 18 was admitted into the chamber and the nitrogen pressure in thechamber was raised to 8 X 10 Electron beam source '12 then was energizedand a resistor film of hafnium nitride was deposited at a rate of 200 to400 A. per minute upon the heated substrate which substrate waspositioned 14 inches from the hafnium source. After deposition of a 4000A. thick hafnium nitride film, evaporation of the hafnium source wasterminated andthe film was heated at 465 C. for 15 minutes in a nitrogenatmosphere in excess of 0.5 torr. The deposited resistor film then wascooled to room temperature and temperature cycle between C. and 125 C.to stabilize the resistor film. Subsequent vmeasurements disclosed thehafnium nitride resistor to have a resistance of approximately 472 ohmsper square and a'temperature coefficient of resistance of -60 p.p.m./ C.between 25 C. and 125 C. X-ray diffraction analysis of a 12,000 A. thickhafnium nitride resistor film disclosed small crystallite HfN presentwith a strong preferred orientation of (111) planes parallel to thesubstrate surface upon which surface the resistor films were deposited.As will be noted from the graph, depicted in FIG. 7, of resistance vs.temperature coeflicient of resistance for reactively evaporated hafniumnitride resistor films, no films, regardless of film thickness, weredeposited having either a positive coefiicient of resistance or aresistance value less than approximately 350 ohms per square. It issupposed that the latter phenomenon results from discontinuities in thesurface layer as the film becomes thicker.

EXAMPLE 5 Titanium nitride resistor films were deposited from a titaniumsource employing the procedure described in the previous examples. Thesubstrate was heated to a temperature of 455 C. prior to deposition andthe titanium nitride was deposited at a rate of 50 A. per minute in anitrogen atmosphere of 8 10 torr. After deposition, the films wereheated for 15 minutes at the deposition temperature in a nitrogenatomsphere greater than 0.5 torr. As can be seen from the graph of FIG.8, the temperature coefficient of resistance values of titanium nitrideresistor films deposited by reactive evaporation change rather rapidlyfrom positive to negative as film thickness is decreased. A resistorfilm having a zero (p.p.m./ C.) temperature coefiicient of resistancebetween 25 C. and 125 C. exhibited a resistance of approximately 270ohms per square upon measurement utilizing the four probe technique.X-ray analysis of the film indicated that random TiN of very smallcrystalline size, e.g. less than 1000 A., was deposited.

EXAMPLE 6 Vanadium nitride films were deposited utilizing the procedureof the previous examples. The nitrogen pressure in the chamber wasmaintained at 8 10 torr, the substrate was heated to 370 C. and thedeposition rate of vanadium nitride upon the substrate was measured tobe approximately 70 A. per minute. Subsequent to the deposition, theresistor film was baked in nitrogen for 15 minutes at a temperature inexcess of 370 C. As can be seen from the graph, e.g. FIG. 8, depictingthe resistance vs. temperature coefiicient of resistance for reactivelyevaporated vanadium nitride resistor films, the temperature coefficientof resistance of the films becomes negative very rapidly as filmthickness decreases. X-ray dilfraction analysis of the deposited filmindicated VN with no preferred orientation of the crystals.

One preferred utilization for the niobium nitride and tantalum nitrideresistor films formed by the reactive evaporation method of thisinvention is in the fabrication of variable resistors of the slidepotentiometer type wherein a wiper arm is traversed a desired distancealong the length of an elongated resistive element. In examining thesuitability of the tantalum nitride and niobium nitride resistor filmsfor utilization in thin film potentiometers, a niobium nitride film wasdeposited in a split annular configuration atop a glass substrate and aspring-loaded commercial sliding potentiometer contact of carbonaceousmaterial was revolved atop the niobium nitride film for 10,- 000revolutions. Resistance measurements of the niobium nitride resistorfilm both before and after rotation of the sliding contact indicated theresistance of the film to be 2800 ohms. A tantalum nitride film then wasdeposited in a split annular configuration upon a glass substrateutilizing the reactive evaporation method of this invention and thespring-loaded carbonaceous contact was revolved atop the depositedtantalum nitride resistor film. No change was observed in an 8,800 ohmtantalum nitride resistor film after 10,000 revolutions.

It was interesting to note that while tantalum nitride and niobiumnitride films showed superior wear resistance to the carbonaceouscontact of the potentiometer, a zirconium nitride film deposited in asplit annular configuration upon a glass substrate by reactiveevaporation showed a variation in resistance from 2400 ohms to 4100 ohmsafter 10,206 revolutions. A second zirconium nitride re sistor filmsimilarly deposited upon a glass substrate by the reactive evaporationmethod of this invention also exhibited a variation in resistance from1500 ohms to 1700 ohms after the commercial sliding contact had revolved10,000 times over the resistor film.

When a commercially utilized film consisting of nickel and 20% chromewas deposited upon the glass substrate and the spring-loaded slidingcontact was revolved over the deposited film for 10,000 revolutions, aresistance variation in the deposited film from a value of 5500 ohmsprior to the initiation of the revolutions of the sliding contact to anohmic value of 8700 ohms after 10,000 revolutions was observed.

While several examples of this invention have been shown and described,it will be apparent to those skilled in the art that many changes andmodifications may be made without departing from this invention in itsbroader aspects; and therefore the appended claims are intended to coverall such changes and modifications as fall within the true spirit andscope of this invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A method of forming thin film resistors comprising positioning asubstrate and a transition metal selected from the group consisting ofniobium, tantalum, vanadium, titanium, zirconium and hafnium within anenclosed chamber, heating said substrate to a temperature in excess of30 C., evacuating said chamber and introducing nitrogen into saidchamber to produce a nitrogen pressure between 5X10 and 10* torr,converting at least a portion of saidtransition metal to the gas phaseby evapora tion of at least a portion of said transition metal anddepositing a thin film transition metal nitride resistor atop saidsubstrate at a rate between 50 and 400 A./minute.

2. A method of forming thin film resistors according to claim 1 furtherincluding baking said nitride resistor at elevated temperatures above 30C. in a nitrogen atmosphere greater than 0.5 torr.

3. A method of forming thin film resistors comprising positioning asubstrate and a transition metal selected from the group consisting ofniobium, tantalum, and vanadium within an enclosed chamber, heating thesubstrate to a temperature in excess of 420 C., evacuating said chamberand introducing nitrogen into said chamber to produce a pressure between5X10 and 10- torr, evaporating at least a portion of the chosentransition metal within said chamber, depositing a thin film transitionmetal nitride resistor atop said substrate, increasing the nitrogenatmosphere within said chamber to a pressure greater than 0.5 torr, andbaking said nitride resistor within said nitrogen atmosphere.

4. A method of forming thin film resistors according to claim 3 whereinsaid transition metal is niobium.

5. A method of forming thin film resistors according to claim 1 whereinsaid transition metal is niobium.

6. A method of forming thin film resistors comprising positioning asubstrate and a transition metal selected from the group consisting ofniobium, tantalum, vanadium, titanium, zirconium and hafnium within anenclosed chamber, evacuating said chamber and introducing a gas selectedfrom the group consisting of nitrogen and ammonia into said chamber toproduce a low pressure nitrogen atmosphere in a range betweenapproximately 5 10 to 10" torr, heating said substrate to a temperaturein excess of 30 C., evaporating said transition metal in said nitrogenatmosphere, and condensing a thin film nitride resistor atop saidsubstrate.

7. A thin film resistor formed by the method claim 1.

8. A method of forming thin film resistors according to claim 1 whereinsaid atmosphere is nitrogen and further including baking said nitrideresistor at elevated temperatures above 30 C. in a nitrogen atmosphereabove at least 0.5 torr.

UNITED STATES PATENTS 3,159,556 12/1964 McLean et a1. 204-56 X 3,181,2095/1965 Smith 117107.1 X 3,242,006 3/1966 Gerstenberg 117--106 X3,315,208 4/1967 Gerstenberg 117-106 X 3,365,692 1/1968 Sartain 338308 XOTHER REFERENCES Holland, Vacuum Deposition of Thin Films, 1956, pp. 109to 114 relied upon.

Holland, Thin Film Microelectronics, 1965, pp. 178 to 182 relied upon.

Hass, Physics of Thin Films, vol. 1963, pp. 1, 55 and 207 to 211 reliedupon.

ALFRED L. LEAVI'IT, Primary Examiner C. K. WEIFFENBACH, AssistantExaminer US. Cl. X.R.

